Influence of microstructure on fracture toughness distribution in ceramic-metal functionally graded materials

This paper deals with the influence of microstructure on fracture toughness distribution in functionally graded materials (FGMs) consisting of partially stabilized zirconia (PSZ) and austenitic stainless steel SUS 304. FGMs and non-graded composites (non-FGMs) with fine and coarse microstructures are fabricated by powder metallurgy using PSZ and two kinds of SUS 304 powders. The fracture toughness is determined by conventional tests for several non-FGMs with each material composition and by a method utilizing stable crack growth in FGMs. The obtained results on the fracture toughness are as follows: (1) The fracture toughness increases with an increase in a content of SUS 304 on both FGMs and non-FGMs. (2) On the fracture toughness of the non-FGMs, the influence of microstructure is negligible. (3) On the FGMs, the fracture toughness is higher in the FGM with fine microstructure than in the FGM with coarse microstructure. (4) The fracture toughness of the FGMs is higher than that of the non-FGMs especially in the case of fine microstructure. Finally, the residual stress in the FGMs created in a fabrication process is estimated from the difference in fracture toughness between the FGMs and non-FGMs.     

Evaluation of R-curve behavior of ceramic-metal functionally graded materials by stable crack growth

This paper deals with the fracture toughness and R-curve behavior of ceramic-metal functionally graded materials (FGMs). A possibility of stable crack growth in a three-point-bending specimen is examined based on the driving force and resistance for crack growth in FGMs, and the distribution of fracture toughness or R-curve behavior is evaluated on FGMs fabricated by powder metallurgy using partially stabilized zirconia (PSZ) and stainless steel (SUS 304). The materials have a functionally graded surface layer (FGM layer) with a thickness of 1 mm or 2 mm on a SUS 304 substrate. Three-point-bending tests are carried out on a rectangular specimen with a very short crack in the ceramics surface. On the three-point-bending test, a crack is initiated from a short pre-crack in unstable manner, and then it propagates in stable manner through the FGM layer with an increase in the applied load. From the relationship between applied load and crack length during the stable crack growth in the FGM layer, the fracture toughness is evaluated. The fracture toughness increases with an increase in a volume fraction of SUS 304 phase.     

K‐dominance of static crack tip in functionally gradient materials

K‐dominance of static crack tip in functionally gradient materials (FGMs) with a crack oriented along the direction of the elastic gradient is studied through coherent gradient sensing (CGS), digital speckle correlation method (DSCM) and finite element method (FEM). In the direction of crack propagation, the shear modulus has a linear variation with constant mass density and Poisson's ratio. First, the CGS and DSCM governing equations related to the measurements and the elastic solutions at mode I crack in FGMs are obtained in terms of the stress intensity factor, material constants and graded index. Secondly, two kinds of FGMs specimens and one homogenous specimen are prepared to observe the influences of the property variation on the K‐dominance. Then, CGS and DSCM experiments using three‐point‐bending of FGMs and homogenous beams are performed. Thirdly, based on the results of the experiments, the stress intensity factors of three kinds of specimens are calculated by CGS and DSCM. Meanwhile, the stress intensity factors are obtained by FEM. Finally, comparing the results from CGS, DSCM and FEM, the K‐dominance of mode‐I static crack tip in FGMs is discussed in detail.     

Neutron and X-ray reflectivity analysis of ceramic-metal materials

Neutron and X-ray reflectivity measurements of a thin film of cermet (ceramic-metal) made by co-sputtering Pt and Al₂O₃ on the surface of a flat piece of float glass are presented. From the analysis of the specular and off-specular measurements, the morphology of the Pt clusters which are embedded in the Al₂O₃ matrix is determined by adjusting a model to the observed data. It is found that the structure of such films presents a certain degree of order in the direction normal to the surface of the films but no correlation (or with a very short correlation length not measurable by this technique) in the plane of the film.     

Interdiffusion reaction in the PZT/PNN functionally gradient piezoelectric ceramic materials

The interfacial diffusion reaction between lead zirconate titanate (PZT) and lead nickel niobate (PbNi1/3Nb2/3O3:PNN) phases in the PZT/PNN functionally gradient piezoelectric ceramics were investigated as a function of the diffusion temperature and time, respectively. The ionic composition distribution profiles in the interdiffusion region were examined by electron probe microbeam analysis (EPMA). Based on a diffusion model of the overlapped diffusion solution from thin slab, the numerical simulation for the ionic composition distribution was carried out by computer, which was in agreement with the EPMA result. The diffusion coefficients for the Ni2+, Nb5+, Ti4+ and Zr4+ ions were determined, which were 33.8, 22.6, 10.8 and 9.9×10-12 m2 s-1, respectively. The apparent activation energies for these ions were 94.4, 171.7, 257.5 and 325.8 kJ mol-1, respectively. The differences in the ionic diffusion coefficients and apparent activation energies were discussed from the viewpoint of the crystal chemistry. © 1998 Chapman & Hall     

Two-dimensional steady heat conduction in functionally gradient materials by triple-reciprocity boundary element method

Homogeneous heat conduction can be easily solved by means of the boundary element method. However, domain integrals are generally necessary to solve the heat conduction problem in the functionally gradient materials. This paper shows that the two-dimensional heat conduction problem in the functionally gradient materials can be solved approximately without a domain integral by the triple-reciprocity boundary element method. In this method, the distribution of domain effects is interpolated by integral equations. A new computer program is developed and applied to several problems.     

Corrosion behaviour and mechanical properties of functionally gradient materials developed for possible hard-tissue applications

Artificial hip joints have an average lifetime of 10 years due to aseptic loosening of the femoral stem attributed to polymeric wear debris; however, there is a steadily increasing demand from younger osteoarthritis patients aged between 15 and 40 year for a longer lasting joint of 25 years or more. Compliant layers incorporated into the acetabular cup generate elastohydrodynamic lubrication conditions between the bearing surfaces, reduce joint friction coefficients and wear debris production and could increase the average life of total hip replacements, and other human load-bearing joint replacements, i.e. total knee replacements. Poor adhesion between a fully dense substrate and the compliant layer has so far prevented any further exploitation. This work investigated the possibility of producing porous metallic, functionally gradient type acetabular cups using powder metallurgy techniques – where a porous surface was supported by a denser core – into which the compliant layers could be incorporated. The corrosion behaviour and mechanical properties of three biomedically approved alloys containing two levels of total porosity (>30% and <10%) were established, resulting in Ti–6Al–4V being identified as the most promising biocompatible functionally graded material, not only for this application but for other hard-tissue implants.     

Fabrication and mechanical properties of glass fibre–carbon fibre polypropylene functionally gradient materials

Hybrid fibre mat reinforced polypropylene (PP) composites with carbon(CF) and glass fibre (GF) were prepared and four kinds of functionally gradient materials (FGM) were fabricated by changing the spatial distribution of GF and CF. To measure the mechanical properties of FGMs and hybrid composites, flexural tests and instrumented impact tests were performed. The flexural strengths and the flexural moduli of hybrid composites increased following the rule of mixture as the relative volume content of CF increased. On the other hand, the total impact absorption energy of hybrid composites decreased with the increment of CF relative volume content. Compared with GF–CF PP isotropic hybrid composite, the composites with compositional gradient showed similar flexural strengths, but characteristic flexural moduli. Especially, sandwich-type FGMs with a CF-rich outer layer and a GF-rich inner layer exhibited higherflexural moduli than others. Total impact absorption energies of four FGMs were also similar, but the ratios of crack initiation energy,Ei, to crack propagation energy, Ep, or ductility index, were quite different.     

Influence of stirring speed on the synthesis of Al/SiC based functionally gradient materials

In the present study, thick aluminum–silicon carbide based functionally gradient materials, with starting weight of 18% SiC were successfully synthesized, using different stirring speeds and an innovative gradient slurry disintegration and deposition technique. The results revealed, in common, an increase in: (a) the weight percentage of SiC particulates, (b) the clustering tendency of SiC, (c) porosity and (d) microhardness, and a reduction in grain size with increasing distance from the base of the functionally gradient material ingots. The results of X-ray diffraction studies indicated no evidence of interfacial reaction products. Furthermore, results revealed an increase in stirring speed increases the homogeneity of SiC particulates' distribution, thus resulting in a decrease in the average gradient of reinforcement along the deposition direction. An attempt is made in this study to establish the trend between processing parameter such as stirring speed with the gradient of SiC particulates realized in the ingots.     

Estimation of the thermal diffusivity profile in functionally gradient materials with the stepwise heating method

Temperature response in a functionally gradient material (FGM) which is subjected to stepwise heating is investigated, to estimate the profile of the thermal diffusivity from the temperature response at the rear surface of the FGM. Emphasis is placed on a distribution parameter which gives the profiles of the thermophysical properties when an exact analytical solution exists for the temperature response in the FGM. An explicit expression to determine the distribution parameter is obtained as a function of the thermophysical properties at the rear surface. This explicit expression can represent the dependence of the temperature response on the thermophysical properties within 5% in comparison to the exact solution. It is expected that this identification can provide useful insight into the estimation of thermophysical properties in FGMs. The usefulness of this relation is also examined by comparing given and estimated profiles for the thermal diffusivity. Fair agreement is demonstrated as far as the trend and the approximate magnitude are concerned.     

Synthesis of Al/SiC based functionally gradient materials using technique of gradient slurry disintegration and deposition: effect of stirring speed

Abstract

In the present study, Al/SiC based functionally gradient materials (FGMs) were successfully synthesised using the gradient slurry disintegration and deposition technique. Gradients of SiC for the starting weight of 18% were successfully established by varying the stirring speed of the molten melt. The results revealed, in general, increases in the weight percentage and clustering tendency of SiC, porosity, and microhardness and a reduction in grain size, with increasing distance from the base of the FGM ingots. The results also showed that an increase in the stirring speed increases the homogeneity of SiC particulates distribution, thus resulting in a decrease in the gradient of reinforcement along the deposition direction. Furthermore, an increase in the stirring speed decreases the number of clusters formed per unit area. An attempt has been made in the present work to establish the trend between processing parameters, such as stirring speed and the gradient of SiC particulates realised in the ingots.     

Synthesis and wear of Al based,free standing functionally gradient materials: effects of different reinforcements

Abstract

In the present study, three aluminium based functionally gradient materials (FGMs), reinforced with different ceramic particulates (silicon carbide, aluminium oxide, and titanium carbide), were successfully synthesised using the innovative gradient slurry disintegration and deposition (GSDD) technique. The results for Al/SiC and Al/Al₂O₃ revealed, in common, an increase in the weight percentage of reinforcement along the direction of deposition, to result in an increase in porosity and microhardness. However, for Al/TiC, the reverse trend was observed, with porosity and microhardness decreasing with increasing distance from the base of the ingot. The porosity levels for Al/TiC were also found to be significantly lower than those ofthe other two FGMs. Thermomechanical analysis of the FGMs showed thatthe average coefficient of thermal expansion of the high reinforcement end was reduced, as compared to the high aluminium end. Sliding wear test results also revealed that the high reinforcement end was more wear resistant than the high aluminium end, except for the case of Al/Al₂O₃. An attempt is made to interrelate the effects of different types of particulates, with microstructural development, microhardness and wear rate results obtained in the present study.     

A design method of Voronoi porous structures with graded relative elasticity distribution for functionally gradient porous materials

International Journal of Mechanics and Materials in Design - FGPMs (functionally gradient porous materials) can satisfy multifold functional constraints with minimizing weights as they are advanced...     

Interface crack problem of functionally graded piezoelectric materials: effects of the position of electromechanical impact and gradient

This paper analyzes the transient response of the dynamic stress intensity factor for an interfacial crack of a functionally graded piezoelectric material (FGPM) coated on the surface of a homogeneous piezoelectric substrate. Different from previous analyses, this study mainly considers a realistic situation when electromechanical loadings are suddenly applied at the material surface. Obtained results are compared with those when the crack surfaces are directly loaded by the same impacts. By using the integral transform method, the problem is reduced to solving two singular integral equations. It is found that dynamic stress intensity factors are significantly amplified and reduced depending on the negative and positive gradient for electromechanical impacts at the material surface. Positive or negative electric impact also decreases or increases the overshoot of the dynamic stress intensity factor. It is suggested that designing an FGPM with a positive gradient index is safer than a negative gradient index.     

低反射高吸收梯度电磁波屏蔽复合材料研究

为了减少反射回空间的电磁波对电磁环境造成的二次污染,本文提出了双梯度电磁屏蔽材料SFGM(shieldingfunc tionallygradientmaterials)设计的构想,来实现对频率<1GHz的电磁波的低反射高吸收。制备的镍/镍锌铁氧体/环氧树脂梯度电磁屏蔽材料,其结果表明:在频率<1GHz时,双梯度材料对电磁波的反射损耗比非梯度材料平均降低了6～8dB;而吸收损耗平均提高了6～14dB。在中心吸收层的衰减常数不变时,对电磁波的吸收损耗随吸收层厚度增加而增加。     

碳/碳/Al2O3陶瓷功能梯度材料的制备与研究

从功能梯度材料的原材料的筛选、制备工艺路线的确定出发,对碳/碳/Al2O3陶瓷功能梯度材料的组分分布、微观结构、烧蚀性能及热学性能进行了研究.从试样内表面向外表面,基体碳含量从88%变化到近乎为15%,而Al2O3陶瓷含量从12%变化到85%左右.材料内表面氧-乙炔烧蚀率为0.012nn/s,这表现为碳/碳材料的特性.材料外表面的导热系数达到0.86W/m·K(25℃),表现为良好的隔热效果.     

Strain gradient elasticity solution for functionally graded micro-cylinders

In this paper, strain gradient elasticity formulation for analysis of FG (functionally graded) micro-cylinders is presented. The material properties are assumed to obey a power law in radial direction. The governing differential equation is derived as a fourth order ODE. A power series solution for stresses and displacements in FG micro-cylinders subjected to internal and external pressures is obtained. Numerical examples are presented to study the effect of the characteristic length parameter and FG power index on the displacement field and stress distribution in FG cylinders. It is observed that the characteristic length parameter has a considerable effect on the stress distribution of FG micro-cylinders. Also, increasing material length parameter leads to decrease of the maximum radial and tangential stresses in the cylinder. Furthermore, it is shown that the FG power index has a significant effect on the maximum radial and tangential stresses.     

Bi-layer functionally gradient thick film semiconducting methane sensors

Gas sensors based on metal oxide semiconductors like tin dioxide are widely used for the detection of toxic and combustible gases like carbon monoxide, methane and LPG. One of the problems of such sensors is their lack of sensitivity, which to some extent, can be circumvented by using different catalysts. However, highly reactive volatile organic compounds (VOC) coming from different industrial and domestic products (e.g. paints, lacquers, varnishes etc) can play havoc on such sensors and can give rise to false alarms. Any attempt to adsorb such VOCs (e.g. by using activated charcoal) results in sorption of the detecting gases (e.g. methane) too. To get round the problem, bi-layer sensors have been developed. Such tin oxide based functionally gradient bi-layer sensors have different compositions at the top and bottom layers. Here, instead of adsorbing the VOCs, they are allowed to interact and are consumed on the top layer of the sensors and a combustible gas like methane being less reactive, penetrates the top layer and interacts with the bottom layer. By modifying the chemical compositions of the top and bottom layers and by designing the electrode-lead wire arrangement properly, the top layer can be kept electrically shunted from the bottom layer and the electrical signal generated at the bottom layer from the combustible gas is collected. Such functionally gradient sensors, being very reliable, can find applications in domestic, industrial and strategic sectors.     

Metal-ceramic functionally gradient material produced by laser processing

A technique of injection with a powder feed of a mixture of metal + ceramic which combines the processes of laser alloying, cladding and injection, has been applied to study the feasibility of using a continuous wave CO₂ laser to produce a functionally gradient material. A 2 kW CO₂ laser has been used to produce, on a nickel alloy substrate, single alloy/clad tracks and three totally overlapping clad tracks using powder mixtures of Al-10 wt% SiC, Al-30 wt% SiC and Al-50 wt% SiC, respectively. The variation of composition and structure with position in the processed material has been investigated with reference to the effect of processing traverse speed and the powder feed rate.On leave from Scientific Research Council, Baghdad.